1. Field of the Invention
The present invention relates to a photoconductor to which high durability and a high image quality during a long term are imparted by disposing in the photoconductor, a photosensitive layer having excellent cleaning properties, high wear resistance and advantageous electrical properties. The present invention relates also to an image forming process, image forming apparatus and process cartridge for the image forming apparatus using the above-noted photoconductor having long life and high performance.
2. Description of the Related Art
Recently, organic photoconductors (OPC) are widely employed in copiers, facsimiles, laser printers, and composite apparatuses thereof owing to excellent performance and various advantages, in place of conventional inorganic photoconductors. Specific grounds thereof are thought as follows: (i) optical properties such as absorbable wavelength and absorption rate, (ii) electrical properties such as higher sensitivity and stable charging ability, (iii) margins of materials, (iv) productivity, (v) lower cost, (vi) safety, and the like.
On the other hand, photoconductors have been small-sized along with image forming apparatuses being small-sized; in addition, higher processing rate as well as maintenance free are have been required for image forming apparatuses; consequently, photoconductors are demanded for higher durability still more nowadays.
However, organic photoconductors are typically less durable since the hardness of the surface layers is relatively low due to their inherent components of charge transport materials of lower molecular mass and inactive polymers; therefore, the surface layers tend to wear significantly due to mechanical stress caused by developing systems and cleaning systems etc. under repeated usages in electrophotographic processes.
Further, rubber hardness of cleaning blades has been raised and pressure onto photoconductors applied from the cleaning blades has been increased so as to improve cleaning ability in order to enhance image quality by using toner particles with smaller particle sizes, which inevitably leading to higher wear rate of photoconductors. The wear of photoconductors certainly degrades sensitivity, electric properties such as charging ability etc., which resulting in deteriorated images such as lower image density and background smear. Further, flaws due to local wear often bring about streak on images due to insufficient cleaning. Such wear and flaws typically dominate photoconductors in terms of lifetime to be exchanged, currently.
As such, the wear rate should be decreased in order to enhance durability of organic photoconductors, which is one of the most important objects in the art.
Previously, various proposals have been provided in order to enhance wear resistance of photosensitive layers, for example, (1) incorporation of curable binders into the photosensitive layer (e.g. Japanese Patent Application Laid-Open (JP-A) No. 56-48637), (2) employment of polymers for charge transport materials (e.g. JP-A No. 64-1728), (3) dispersing inorganic fillers into surface layers (e.g. JP-A No. 4-281461), and the like.
However, in the (1) incorporation of curable binders described above, residual voltage tends to increase owing to impurities such as polymerization initiators and/or unreacted residual groups due to insufficient compatibility with charge transport materials, thus image density tents to decrease. In the method (2) using a charge transport polymer and the method (3) using an inorganic filler, while the wear resistance of the photoconductor can be improved to some extent, a photoconductor which can fully satisfy the durability required for the organic photoconductor is not yet obtained. Further, in the method (3) using an inorganic filler, the organic photoconductor comprising an inorganic filler has such a tendency that due to a charge trap which is present on the surface of the inorganic filler, the residual electric potential of the surface layer is elevated, so that the image density is easily lowered. As such, based on these proposals (1), (2), and (3), the durability of organic photoconductors is not satisfactory on the whole, including electrical durability and mechanical durability.
Further, photoconductors containing cured product of a multi-functional acrylate monomer are proposed in order to improve the abrasion resistance and scratch resistance such as of (i) (e.g. Japanese Patent No. 3262488). In the patent literature, it is disclosed that cured material of the multi-functional acrylate monomer is included into a protective layer on photosensitive layers. However, there exist no more than simple descriptions that a charge transport material may be contained in the protective layer and there exist no specific examples. Further, when a charge transport material having a low molecular mass is simply added to the surface layer, it may cause problems related with the compatibility to the cured body, thereby crystallization of charge transport material having a lower molecular mass and clouding may occur, resulting in reduction in mechanical properties.
In addition, a photoconductor is produced by way of causing reaction of monomers in a condition that a polymer binder is incorporated; therefore, there will be some problems that the curing cannot sufficiently proceed, and surface nonuniformity is induced due to phase separation at curing caused by insufficient compatibility between the cured material and the binder resin, which resulting in inferior cleaning in image forming apparatuses.
Further, another proposal is disclosed for reducing abrasion wear of photosensitive layers, in which a charge transport layer is provided using a coating liquid that comprises a monomer having a carbon-carbon double bond, a charge transport material having a carbon-carbon double bond, and a binder resin (e.g. Japanese Patent No. 3194392). The binder resin includes a binder reactive with the charge transport material having a carbon-carbon double bond and another binder non-reactive with the charge transport material without having the double bond. The photoconductor allegedly represents higher wear resistance as well as proper electrical properties. However, non-reactive resins as the binder resin tend to yield surface irregularity and thus inferior cleaning, since the non-reactive resins are typically non-compatible with reaction products between the monomer and the charge transport material, thus phase separation is likely to occur. Further, the patent literature discloses monomers having two functionalities as specific examples, which cannot bring about sufficient crosslinking density and satisfactory wear resistance due to the lower functionalities. Provided that reactive resins are employed as the binder resin, the bonding density and the crosslinking density are possibly not sufficiently high due to the lower functionalities of the monomer and the binder resin, thus electrical properties and wear resistance will not be satisfactory.
Further, another proposal is disclosed, in which photosensitive layers comprise reaction products that are produced by curing hole transport compounds having two or more functional groups capable of undergoing chain polymerization in a molecule (e.g. JP-A No. 2000-66425). However, the photosensitive layer tends to cause higher internal stress and thus to yield higher surface roughness and cracks, since the bulky hole transport compound have two or more chain polymerizable functional groups.
Further, even when the wear resistance of the photoconductor is improved, it does not mean the obtaining of the high durability of the photoconductor that a cleaning failure, an abnormal image, such as an image blur and an image degradation are easily caused due to the above-noted improvement of the wear resistance. Particularly with respect to a polymerized toner (or a toner having the form of a sphere) which is frequently used in the industry recently, when the toner is removed by the cleaning from a photoconductor having a high surface energy (or high friction coefficiency), a cleaning failure due to the escape of the toner from the cleaning blade is easily caused.
For lowering the surface energy or the friction coefficiency of the photoconductor surface, a method in which various lubricants are incorporated in the composition of the surface layer of the photoconductor is known. A method in which in the composition of the surface layer of the photoconductor, a lubricant, such as a fluorine-modified silicone oil is incorporated, is disclosed (see JP-A No. 07-295248, 07-301936 and 08-082940). In this method, the effect for the cleaning properties of the photoconductor or the removing of a contaminator from the photoconductor can be confirmed by lowering the surface energy of the photoconductor. However, since a fluorine-modified silicone oil is transferred closely to the surface of the photoconductor during the disposing of the protective layer, due to a slight wear of the surface layer during the repeated using of the photoconductor, the effect of the fluorine-modified silicone oil is lost in an early period of using the photoconductor. Therefore, it has been the actual state that even by using the fluorine-modified silicone oil, a satisfactory effect for enhancing the durability of the photoconductor cannot be obtained.
Further, with respect to the method for incorporating various lubricants in the form of fine particles in the composition of the most outer surface layer of the photoconductor, various attempts have been performed. Examples of the lubricant incorporated in the composition of the most outer surface layer of the photoconductor include fine particles of a silicone resin, fine particles of a resin containing a fluorine atom (see JP-A No. 63-65449) and fine particles of a melamine resin (see JP-A No. 60-177349). Examples of the above-noted method include a method in which polyethylene resin particles are incorporated in the composition of the surface layer (disclosed in JP-A No. 02-143257), a method in which particles of a resin containing a fluorine atom are incorporated in the composition of the surface layer (disclosed in, for example JP-A No. 02-144550), a method in which silicone resin fine particles are incorporated in the composition of the surface layer (disclosed in, for example JP-A Nos. 07-128872 and 10-254160) and a method in which cross linkable resin fine particles are incorporated in the composition of the surface layer (disclosed in, for example JP-A No. 2000-010322 corresponding to U.S. Pat. No. 5,998,072). Further, a method in which methylsiloxane resin fine particles are incorporated in the composition of the surface layer is disclosed in, for example JP-A No. 08-190213. The method in which the above-noted lubricants in the form of the fine particles are dispersed in the surface layer of the photoconductor is effective for enhancing the persistence of the effect as the lubricant and is more effective for enhancing the durability of the photoconductor than the above-noted method in which a silicone oil or the like is incorporated in the composition of the surface layer of the photoconductor. However, with respect to such a photoconductor, since the lubricant is incorporated in the charge transport layer having unsatisfactory wear resistance, while the lubricant has the effect of suppressing the attaching of various substances to the photoconductor in an early using of the photoconductor, the persistence of such suppressing effect for a long term is unsatisfactory.
Thus, it cannot be mentioned nowadays that a photoconductor comprising a crosslinked photosensitive layer which comprises the above-noted conventional lubricants has satisfactory general properties.